Process for preparing alkali metal oxides and peroxides



PROCESS FOR PREPARING ALKALI METAL OXIDES AND PEROXIDES George L. Cunningham, Cleveland Heights, Ohio, assignor to Horizons Incorporated No Drawing. Application December 16, 1954, Serial No. 475,843

3 Claims. (Cl. 23-184) This invention relates to a new and useful process for the preparation of metallic oxides. More particularlyit relates to a process whereby metallic oxides, peroxides and superoxides may be inexpensively prepared in the form of pure compounds.

Many processes have been suggested for the preparation of oxides of the various metals. Generally speaking, one common approach involves the direct oxidation of the metal. An obvious disadvantage of methods based on direct oxidation of the metal is that to be commercially attractive, the metal itself must be available at a reasonable price. Another suggested approach is the decomposition of oxygen containing compounds such as carbonates, nitrates, sulphates, phosphates, oxalates, tartrates and the like, either with or without the presence of reducing agents such as carbon. Such methods are of limited application, and have not been found suitable for the direct preparation of the oxides of most metals. Still another approach involves electrolytic production of the metals followed by oxidation of the metal. Still another procedure which has been suggested is the oxidation of relatively dilute amalgams.

One object of my invention is to produce the oxides, peroxides or superoxides of various metals by a simple process not subject to the disadvantages present in the prior art.

A more specific object of my invention is to produce the oxides, peroxides and superoxides of some of the alkali ates atom" metals by a process which does not employ the alkali metal, in the form of a pure metal, per se.

These and other objects are accomplished by a process in which an amalgam is prepared containing the metal whose oxidic derivative is to be produced and the amalgam is then oxidized under carefully controlled conditions by direct contact with oxygen or an oxygen containing gas.

in a preferred embodiment of my invention a relatively dilute amalgam is obtained by electrolysis of a compound of the metal whose oxidic derivative is sought and this dilute amalgam is added to a reservoir of a more concentrated amalgam which is then oxidized and from which the oxide derivative is readily removable.

While the method of my invention is applicable generally to the formation of most, although not all, of the oxidic derivatives of the metals, it has been found to be particularly useful for the preparation of oxidic compounds of the alkali metals. Since the preparation of sodium oxide, sodium peroxide and sodium superoxide may be taken to be representative of the practice of my invention, the process will be hereinafter specifically de-, scribed with reference to these three compounds, it being understood that except for incidental variations in the specific process conditions (e. g., higher temperatures,

different concentrations, etc.), this description applies with 1 equal force and effect to the preparation of oxides, peroxides and superoxides of the other alkali metals generally insofar as they are amenable to the described treatments.

At least three distinct methods have been proposed for the preparation of the oxides of sodium. The first, direct oxidation of the metal in air or oxygen produces both the oxide and peroxide, but production of the superoxide requires prolonged oxidation by means of gaseous oxygen at high temperatures and pressures. The objection to such procedures is the requirement of metallic sodium which must be produced by an expensive electrolysis. A

proposed method, the dehydration of sodium hydroxide has not been found to be feasible. A third suggested method which is disclosed in U. S. Patent 1,961,160, is based on the distillation of dilute sodium amalgams to produce pure sodium which is then oxidized by means of pure oxygen. it has been found virtually impossible to obtain pure sodium by this process because of several operational obstacles inherent in the patented process. The method involves the distillation of relatively large amounts of metallic mercury per unit of sodium produced. The separation is rendered difiicult because the vapor pressure of mercury decreases in a verymarked fashion with relatively slight increase in the concentration of the sodium. Thus, in very concentrated amalgams, the vapor pressure of mercury is very low and when the temperature is raised the vapor pressure of the sodium approaches that of the mercury which renders the removal of meirury alone difiicult and almost impossible. Accordin ly, sodium and the other alkali metals made in this fashion are contaminated with considerable amounts of mercury, and the method has not found a wide acceptance.

Since the aforementioned patented process discloses that when an amalgam is distilled, two liquid layers are obtained, and the published phase diagrams for the alkali metal-mercury systems show no regions of concentrations or temperatures where two liquid layers would exist, the inability of the process to produce pure alkali metal is apparently inherent therein.

I have found a method whereby most oxidic derivatives of the alkali metals can be prepared at relatively low temperatures in a relatively simple manner. According to the method I have invented, a pool of alkali metal amalgam is formed in any convenient manner, in which the alkali metal concentration is chosen after consulting the alkali metal-mercury system diagram, so as to be higher than the concentration at which a high melting solid product exists. In the case of sodium, for example, the Na-Hg system published in Mellors Treatise on Inorganic and Theoretical Chemistry indicates that at about 5% sodium, a compound melting at about 353 C. exists, corresponding to Nal-Igz.

maximum-dystectic-point being shown at about 50 atomic percent for lithium (LiHg), 25 atomic percent for potassium (Bldg), 33 atomic percent for cesium (CsHgz) and 14 atomic percent for rubidium (R'bHgs). The pool of amalgam of the desired concentration is prepared in any convenient manner. One convenient method is to plunge pieces'of alkali metal, held in an inverted cup, under the surface of a body of mercury, in which the pieces will readily dissolve. Another method by which the pool of amalgam may be prepared is by the carefully controlled distillation of dilute amalgams. Whatever the manner in which it is formed, in accordance withmy invention,

there is fed into the prepared pool of amalgam, either method. For example, sodium amalgam is presently The phase diagrams for theother alkali metals exhibit a similar configuration, the.

teeper made commercially by the electrolysis of a substantially saturated solution of sodium chloride at about 90 C., using a graphite or platinum anode and a mercury cathode. Chlorine and sodium amalgam formed at the anode and cathode respectively, are recovered. in accordance with present practice such a cell operated at about 4 volts (in comparison with about 7 volts in a fused sodium chloride cell) will have a current eiiiciency of about 95%. "Corrosion of the apparatus is negligible. The cell operates with brines which may be relatively impure, and over 95% of the sodium chloride in the brine can be converted into chlorine and sodium amalgam. Other cells, e. g. a cell employing aqueous sodium hydroxide could be electrolyzed whereby oxygen as well as the desired amalgam could be obtained.

The sodium amalgam produced in such cells usually contains about 0.1% sodium (by weight). At slightly lower efliciencics, amalgams with up to about 0.5% (by weight) of sodium can be produced. The production of amalgams with more than 1% sodium is not generally practiced because the eiiiciency of the cell is materially reduced. Furthermore, the formation of a-productwhich is solid at the 'cell temperature places an upper limit on the amountof sodium which the amalgam can contain, as may be readily seen from the sodium-mercury phase diagram.

It 'has been proposed to form the oxides of the alkali metals by oxidation of the dilute amalgam. With very dilute amalgams such as those above described, separaration of the relatively minute amounts of sodium oxide or peroxide from the large amount of mercury has been found to be difiicult. Furthermore, the presence of the excess mercury tends to decompose some of the sodium oxide or peroxide. Reaction rates are slow. When amalgams'having the desired concentration of alkali metal are oxidized in accordance with my invention, that is, with simultaneous addition of a dilute amalgam, these difficulties are avoided. From the sodium-mercury phase diagram it will be seen that at about sodium (by weight) the amalgam has a maximum melting point of approximately 353 C. At this point a solid-having the composition NaHgz is the solid phase. Accordingly the dilute amalgam is added to an amalgam of such concentration that the composite always possesse a concentration somewhat greater than 5% sodium, er g. more than about 7% sodium by weight and preferably-greater than about 10% sodium whereby the formation of undue amounts 'of any solid phase and the difiiculties this entails are avoided. Thus, when the sodium amalgam is concentrated in accordance with my invention, the possibility of forming a thick amalgam containing solid NaHgz is minimized.

In accordance with my invention a dilute sodium amalgam is prepared by electrolysis in a manner well known in the art. 'It-may be produced in an electrolytic cell such as that described in the aforesaid Moulton Patent 1,961,160, into which the electrolyte is fed through an interrupter to avoid short circuiting the cell and from which the amalgam is withdrawn, through another interrupter. A pool of more concentrated sodium amalgam is also prepared, with a sodium content of preferably about-10% by weight, although in some instances, it may be found advantageous to employ amalgarns with a higher concentration of sodium. The prepared amalgam is charged into a mercury still, provided with'means to 'admit the dilute amalga'rneither continuously or intermittently. One particularly suitable arrangement is to-shower the dilute amalgam into the vapor space in the still from a point near the top of the still. The still-is maintained under reduced pressure by means of a vacuum pump. The same pump is advantageously employed to compress the mercury vapor drawn ofi from the still, which 'will condense at elevated temperatures dependent upon the 4 still, by any suitable type of heat exchanger. In this way the heat of vaporization of the mercury may be largely recovered.

The still in which the pool of combined amalgams is being distilled also serves as the vessel in which the sodium or other alkali metal is oxidized. The oxidation is effected by means of a suitable oxidizing agent, preferably air or oxygen, which must be carefully added "to avoid the oxidation of the mercury. Mercury can be distilled at tempera tu're's of about 350 C. in the presence of air Without any appreciable oxidation of the mercury. In the presence of even trace amounts of sodium or other alkali metals, mercury under otherwise similar conditions is rapidly oxidized. I

I have found that in spite "of the aforesaid difficulties, the alkali metal in the amalgam may be controllably oxidized to form the oxide, peroxide or, in the case of potassiui'n, cesium or rubidium, the superoxide, by carefully controlling the factors influencing the reaction, particularly the temperature, oxygen partial pressure and alkali metal concentration. By proper control of these variables, oxidation of the mercury may be prevented or any mercury which may be oxidized, may be decomposed as "rapidly as it forms.

The relative ease with which sodium reacts with *oxygen prevents the system from approaching a condition such that mercury would be in equilibrium with oxygen and an oxide of mercury, but in the event that the oxide did form, it would be readily perceived because of its red-yellow color, it would only be necessary to lower the oxygen pressure, or raise the temperature, or otherwise alter the operating conditions, so that the reaction ZHgOZZHg-l-Oz proceeds to the right instead of the left.

In practice the oxygen admitted to the system is caused to enterthe mercury still at the surface of the heated amalgam, or at a point slightly below the surface. The heat of formation of the'oxidic derivative of sodium, or other metal, is thus efiectively recovered at a point where it assists in the distillation of the mercury without incurring any tendency to spatter or bump in the amalgam. Sincethe specific gravities of the various oxidic derivatives are all considerably lower than the specific gravity of the amalgams, the oxidic derivatives will float on the surface, from which they can be readily removed by mechanical means, such as a skimming tool. The following example will serve to further illustrate the practice of my invention under one set of operating conditions. Obviously those skilled in the art will appreciate that to produce other-oxygen derivatives of other metals, the operating conditions may be suitably modified, as necessary, without departing from the intended scope of niy invention.

. Example 'Aipool'of sodium amalgam containing approximately 10% sodiumby weight (49.2 mol percent Na) was prepared by feeding lumps of metallic sodium-into mercury.

"To this pool, 11,500 parts of dilute sodium amalgam previously prepared in a conventional electrolytic cell employing a mercury cathode was continuously added. The dilute amalgam contained about 0.4% sodium, by weight. A vacuum of about '50 millimeters of mercury Y and a temperatureof about 450 C. was maintained in thesiill. "While approximately 11,450 parts by weight of mercury were distilled an and recovered, 78 parts of sodium-peroxide were produced which were scraped off the pool and recovered substantially free from any con- *tamination.

pressure. ,The condensed mercury may be employed =-to heat the dilute amalgam prior to its admission into the 'In theforegoing operation itshould be noted that the stray currents are to be avoided in the electrolytic production of the dilute amalgam. This may be accomplihedby employingan-intermittent feed for the sodium amalgam and the mercury returned to the electrolytic cell after distillation and condensation of same is handled in like manner, i. e. as semicontlnuous streams, enabling the entire operation to be conducted continuously.

It will be observed that except for the metal employed when the initial pool of amalgam is prepared, the foregoing process does not require the use of the expensive alkali metal as such, but instead converts readily prepared and inexpensive dilute amalgams into products of high purity for which commercial uses are widespread.

I claim:

1. A process for preparing OXidic compounds from the group consisting of monoxides and peroxides of an alkali metal which comprises: preparing a relatively dilute amalgam of said metal, having a content of less than 0.5 by weight of said metal; forming a pool of a concentrated liquid amalgam of a metal from the group consisting of alkali metal amalgams in which the concentration of the said metal is substantially greater than the concentration corresponding to the said metal-mercury compound with a maximum melting point; forming a composite liquid amalgam wherein the relative amounts of dilute liquid amalgam and concentrated liquid amalgam are chosen to insure that the resulting concentration of the said metal in the composite liquid amalgam is substantially greater than the concentration corresponding to the metal-mercury compound with a maximum melting point by introducing the dilute liquid amalgam into the said pool of concentrated amalgam; maintaining the concentration of the metal in the composite liquid amalgam greater than the concentration corresponding to the metal-mercury compound with a maximum melting point by distilling mercury from the liquid pool at a rate not substantially in excess of the rate at which mercury is introduced into the pool as dilute amalgam; oxidizing the mixture of 3 amalgams to produce the desired oxidic compound and recovering the oxidic compound.

2. The process of claim 1 in which thedilute amalgam is continuously introduced into the composite liquid amalgam.

3. A process for preparing sodium peroxide which comprises: preparing a relatively dilute sodium amalgam containing up to 0.5% by weight of sodium; forming a pool of a concentrated liquid amalgam of sodium, containing more than 7% sodium by weight; introducing the dilute liquid amalgam into the said pool of concentrated amalgam to form a composite liquid amalgam in which the relative amounts of dilute and concentrated amalgam are proportioned to insure that the resulting concentrtaion of the sodium is greater than the concentration correspond-iug to NaHgz; maintaining the concentration of the sodium in the mixture of amalgams greater than the concentration corresponding to NaHgz by distilling mercury from the liquid pool at a rate not substantially in excess of the rate at which mercury is introduced into the pool as dilute amalgam, oxidizing the mixture of amalgams to produce sodium peroxide and recovering the sodium peroxide.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES I. W. Mellor: A Comprehensive Treatise on Inorganic and Theoretical Chem, vol. 4, 1923 ed., pp.

5 1010-1018 inclusive and pp. 10304034, Longmans,

Green & Co., New York. 

1. A PROCESS FOR PREPARING OXIDIC COMPOUNDS FROM THE GROUP CONSISTING OF MONOXIDES AND EROXIDES OF AN ALKALI METAL WHICH COMPRISES: PREPARING A RELATIVELY DILUTE AMALGAM OF SAID METAL, HAVING A CONTENT OF LESS THAN 0.5% BY WEIGHT OF SAID METAL; FORMING A POOL OF A CONCENTRATED LIQUID AMALGAM OF A METAL FROM THE GROUP CONSISTING OF ALKALI METAL AMALGAMS IN WHICH THE CONCENTRATION OF THE SAID METAL IS SUBSTANTIALLY GREATER THAN THE CONCENTRATION CORRESPONDING TO THE SAID METAL-MERCURY COMPOUND WITH A MIXIMUM MELTING POINT; FORMING A COMPOSITE LIQUID AMALGAM WHEREIN THE RELATIVE AMOUNTS OF DILUTE LIQUID AMALGAM AND CONCENTRATED LIQUID AMALGAM ARE CHOSEN TO INSURE THAT THE RESULTING CONCENTRATION OF THE SAID METAL IN TH COMPOSITE LIQUID AMALGAM IS SUBSTANTIALLY GREATER THAN THE CONCENTRATION CORRESPONDING TO THE METAL-MERCURY COMPOUND WITH A MAXIMUM MELTING POINT BY INTRODUCING THE DILUTE LIQUID AMALGAM INTO THE SAID POOL OF CONCENTRATED AMALGAM; MAINTAINING THE CONCENTRATION OF THE METAL IN THE COMPOSITE LIQUID AMALGAM GREATER THAN THE CONCENTRATION CORRESPONDING TO THE METAL-MERCURY COMPOUND WITH A MAXIMUM MELTING POINT BY DISTILLING MERCURY FROM THE LIQUID POOL AT A RATE NOT SUBSTANTIALLY IN EXCESS OF THE RATE OF WHICH MERCURY IS INTRODUCED INTO THE POOL AS DILUTE AMALGAM; OXIDIZING THE MIXUTRE OF AMALGAMS TO PRODUCE THE DESIRED OXIDIC COMPOUND AND RECOVERING THE OXIDIC COMPOUND. 